Abstract: Due to the objective restrictions and impact in the process of antenna gain measurement,the test data are in small samples and poor information. Sometimes the processing result is inconsistent with the practical condition if all parameters are treated as definite numbers in deterministic computation. So the data processing method for the antenna gain measurement based on unascertained theory is put forward. The uncertainty of the parameters is expressed by unascertained rational number and the constitution method for unascertained rational number of test data is introduced. A practical antenna gain is computed and analyzed by certainty and uncertainty method. The comparison result shows that unascertained rational number concern better on the uncertainty of computation parameters and could avoid the absolutization of certainty expression.The computation result is more reliable and reasonable compared with that of traditional method.